A mode-suppressing waveguide-to-coax transition which is useful for feeding a high powered pylon (slotted-cylinder) broadcast antenna.
Terrestial television broadcasters derive their incomes by revenues associated with the broadcasting of television signals to television receivers within a certain region. Each broadcaster in order to maximize his income attempts to reach the maximum possible number of viewers. For each television broadcasting site, there is a maximum permissible transmitted power, which depends upon the frequency of operation. At VHF frequencies, the current maximum permissible power is less than 100 killowatts (kw) while at UHF frequencies (470-830 MHz) the maximum permissible peak-of-sync power is in excess of 200 kw. The maximum power is used when viewers at substantial distances from the broadcasting site are to be reached. In order to benefit from transmitter power near the maximum allowable power, the broadcast antenna should be placed at the highest possible location, as for example atop a hill or tower (otherwise the signal energy at nearby viewers exceeds the minimum necessary for good reception, and the excess power is wasted). Such towers may be up to 2,000 feet in height in order to give the best line-of-sight to the distant viewers. In order to efficiently direct the radiated power towards the most distant viewers, high-power television broadcast arrangements ordinarily use a high-gain antenna, which concentrates most of the energy into a solid angle only a few degrees in height, generally centered on the horizon. Such high-gain antennas require a large vertical aperture in order to achieve the high gain necessary to so concentrate the energy. The large vertical aperture requires a structure having a vertical dimension or height of as much as 32 wavelengths. Since a wavelength at lower UHF frequencies is more than two feet, it can be seen that the vertical height of a high-gain UHF antenna may exceed 60 feet.
The environmental conditions at the antenna location are very severe. The antenna may be subjected to lightning, severe winds due to the high and unobstructed location atop the tower, and icing. These environmental conditions place severe mechanical restrictions on the antenna. The high winds, especially in the presence of an ice load, create an upsetting force or moment which tends to twist the antenna from the top of the tower. The magnitude of the forces increases as the square of the distance which the antenna projects from the support. If guy wires could be used to support the top of the antenna against the lateral component of the forces due to wind loading, the mechanical requirements would be eased. However, due to the omnidirectional nature of the radiation desired from the antenna in ordinary applications, the perturbing presence of guy wires is not acceptable. Consequently, the antenna is supported in cantilevered fashion from the top of the tower. In order to minimize the magnitude of the forces which must be resisted in a cantilevered fashion near the base of the antenna, the wind loading of the antenna must be reduced as much as possible. A very popular type of UHF antenna having low wind loading and a sturdy structure is the slotted-cylinder or pylon antenna (also known as a beacon antenna) which includes an elongated vertical transmission line having a conductive outer surface into which radiating slots are cut. The energy passing through the transmission line is coupled to the slots in a controlled fashion to produce the desired antenna array factor, and any energy continuing past the last slot is either dissipated in a resistive load or controllably reflected for radiation by the slots. The transmission-line may be either waveguide or coaxial (coax). A circular waveguide has lower loss than a coaxial transmission line, but has a larger diameter. The smaller diameter of the coax gives reduced wind loading, and the pylon antenna generally uses a coaxial transmission-line feed.
As mentioned, the antenna location is at the top of a tower which may be as much as 2,000 feet tall. The transmitter or power amplifier which produces the television signal energy is a large structure which is located on the ground. The signal energy must be transmitted from the transmitter, up the tower to the feed point at the base of the antenna. As mentioned, waveguide has lower loss than coax. Because of the great length of the run from the transmitter to the base or feed of the antenna, it is very desirable to use waveguide. This becomes apparent when considering that a difference of only 0.1 dB in transmission loss represents about 7000 watts difference in loss (for an input power of 200 kw). The larger dimensions of the waveguide are not a crucial consideration for the run up the tower, because the lateral forces due to wind loading on the tower and on the waveguide may be resisted by guy wires attached to the top of the tower (the base of the antenna). A waveguide-to-coaxial-transmission-line adapter (waveguide-to-coax-adapter) is required between the output of the waveguide and the coaxial feed of the antenna.
The power handling capability of a coaxial transmission line depends upon its dimensions. For a given impedance level, as for example 50 ohms, the higher the power being carried, the higher the voltage between the inner and outer conductors and the larger the current flow in the center and outer conductors. High power operation therefore requires large inner-to-outer conductor spacing to prevent voltage breakdown and large cross-sectional dimension of the inner conductor to reduce I.sup.2 R losses. It has been found that for UHF power levels in the vicinity of 200 kw that reliable operation of the coax requires an outer conductor diameter of 10 to 12 inches. At such diameters, a coaxial transmission line supports not only the dominant TEM mode propagation but will also support TE.sub.1,1 mode propagation at the frequency of operation. Propagation of the TE.sub.1,1 mode is undesirable because of the possibility of affecting the amount of power coupled into the various radiating slots and thereby affecting the aperture distribution of the antenna. This can result because the slots are fed with two modes TEM and TE.sub.1,1, one of which is not controlled. FIG. 8a illustrates the TEM mode, and FIG. 8b illustrates the TE.sub.1,1 mode. Higher-order modes which can be propagated in coax are illustrated in FIGS. 8c-d.
The undesirable TE.sub.1,1 mode may be suppressed by using for the waveguide-to-coax transition the apparatus described in U.S. Pat. No. 2,878,453 issued Mar. 17, 1959 to Elliott. The Elliott arrangement includes a rectangular waveguide which gradually splits, and the resulting split waveguides twist in opposite directions until they are recombined to form a coax in a manner which produces TEM mode propagation in the coax while supressing TE.sub.1,1 mode operation. However, the Elliott transition is physically large and very difficult to fabricate, especially in view of the required longitudinal twist to the rectangular waveguide and because of the large size of the waveguide at lower than UHF frequencies. Another waveguide-to-coax transition is described in U.S. Pat. No. 2,619,539 issued Nov. 25, 1952 to Fano. The Fano arrangement includes a coaxial line which intersects the broad wall of a rectangular waveguide, with the center-conductor passing through the waveguide to the opposite wall of the waveguide. A conductive plate lying parallel to an axis of symmetry of the waveguide connects the center and outer conductors inside the coax. The bandwidth of this structure is narrow because the waveguide is in effect short-circuited by the conductive plate and the center conductor of the coax. The narrow bandwidth is undesirable for use over a television channel, where the bandwidth is 6 MHz.
Thus, high-power television broadcasting arrangement is desired in which the antenna is coaxially fed and in which a run of waveguide couples the high signal power to the coax with suppressed TE.sub.1,1 mode.